White light emitting units, or colour tuneable emitting units having arrays of coloured LEDs, such as RGB LED luminaries and LED displays, are of interest for several reasons. For example, they are low cost and efficient, and the chromaticity of their light output is adjustable.
However, the light output of the LEDs varies from LED to LED, as well as over the life time of each LED. Additionally, the light output of the LEDs varies inversely with temperature, and the variation is different for different colours. The varying output of a LED is also a problem in single LED emitting units. Many solutions for achieving a stable light output from emitting units have been tried. In those solutions different kinds of feedback control have been introduced. Thus, the light output is detected and the detections are used for controlling the energising of the LEDs. One prior art solution is disclosed in U.S. Pat. No. 6,495,964, where the light output of the emitting unit is detected for different colour combinations, by turning off the LED(s) for the colour to be detected. Thus, several detection pulses are generated sequentially during a time period, while performing a coordinated sequential turn off of LEDs emitting light of different colours.
This and other prior art solutions will work fairly well in a substantially dark environment, where the emitting unit is the exclusive light source. However in most practical cases this assumption does not hold. Also a part of the environmental light is usually able to reach the detector embedded in the emitting unit. This contribution has to be discriminated in the feedback part of the control system from the light emitted by the LEDs themselves in order to prevent colour or luminance errors. This is of particular importance in applications with strong requirements on chromaticity and stability of colour point in time. This discrimination is for example done by subtraction. Depending on the application and the specific situation in which such LED emitting units are used, standard continuous background subtraction does not give sufficiently good results. By continuous background subtraction means detecting any remaining light while all LEDs are turned off. In particular pulsed light sources, such as TL tubes, monitors or other LED modules, in the neighbourhood can cause troubles. Assume, for example, that a TL tube accidentally always emits while the red light contribution is measured. Then the detector will see a distorted light, which will cause the provision of wrong information to the control circuitry, which will, in turn, cause an erroneous setting of the colour mix.